[Haskell-cafe] Expanding do notation
josef.svenningsson at gmail.com
Sat Jan 7 12:23:52 EST 2006
On 1/7/06, Chris Kuklewicz <haskell at list.mightyreason.com> wrote:
> When you put "print (head p)" at then end, it keeps a reference to the
> whole list "p" which is your space leak. If you want to store the head
> of p, this *should* work:
> > main = do n <- getArgs >>= return . read . head
> > let p = permutations [1..n]
> > headOfP <- return (head p)
> > mapM_ (putStrLn . concatMap show) $ take 30 p
> > putStr $ "Pfannkuchen(" ++ show n ++ ") = "
> > putStrLn . show $ foldl' (flip (max . steps 0)) 0 p
> > print headOfP
> The "headOfP" is computed as an IO action at that point in the program,
> so "headOfP" does not hold a reference to "p" as a whole.
Without having tried this I'd say that you should use
Control.Excection.evaluate instead of 'return'. Or you could use 'seq'. I
suspect that otherwise the 'head p' will not be evaluated until the last
line and you will have the same problem as before.
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